https://gitlab.com/warsaw/flufl.testing

"As of 2020-06-17 I have archived this project and will no longer be developing it."

The only usage of python3-flufl.testing is as an autopkgtest dependency of mailman3.

Maybe it can be patched-out.

python3-flufl.testing is hindering the removal of python3-pkg-resources

Greetings

Alexandre